The present invention relates generally to the separation of gaseous mixtures, and, more particularly, to the separation of gaseous mixtures using acoustic waves.
The spectrum of practical applications requiring separation of mixtures is broad, including large-scale industrial processes such as petroleum refining, air separation, and beverage processing, and smaller-scale processes such as isotope separation and chemical analysis. A large number of xe2x80x9cphysicalxe2x80x9d (i.e., not chemical) mixture-separation techniques are well understood and in widespread use, including time-independent thermal diffusion, gaseous diffusion, fractional distillation, centrifugation, electromagnetic separation, and chromatography.
Of these, distillation is the most widely used in the large-scale petroleum refining and air separation industries, and large distillation plants have efficiencies approaching half the efficiency limit imposed by the laws of thermodynamics. However, many mixtures cannot be practically separated by distillation, usually because the components of such mixtures have equal or nearly equal boiling points. Mixtures of isotopes or mixtures of isomers must usually be separated by less efficient, non-distillation methods, such as time-independent thermal diffusion using Clusius-Dickel columns. When the two components of the mixture have nearly identical densities, however, the gravity-dependent aspects of Clusius-Dickel columns fail, and even more awkward and/or inefficient separation methods must be used.
Even with this broad spectrum of existing methods, some separations are still difficult or impossible, for reasons such as instability of the mixture""s molecules at elevated temperature, exact equality of the components"" densities, freezing of the mixture at reduced temperatures, and safety considerations demanding low mixture inventory.
Swift et al., xe2x80x9cThermal diffusion and mixture separation in the acoustic boundary layer,xe2x80x9d 106 J. Acoust. Soc. Am. 4, Pt 1, pp. 1794-1800 (1999) reports a mixture separation method based on thermoacoustic phenomena. The separation mechanism results from two simultaneous interactions of a sound wave in a gas with a solid boundary aligned parallel to the sound-propagation direction. A thin layer of the gas mixture adjacent to the solid boundary is immobilized by viscosity while the rest of the gas mixture moves back and forth with the sound wave. The heat capacity of the solid boundary holds this thin layer of the gas mixture at constant temperature while the rest of the gas mixture experiences oscillations in temperature due to the oscillating pressure of the sound wave.
The oscillating temperature and attendant oscillating thermal diffusion cause the two components of the gas mixture to take turns diffusing into and out of the immobilized layer, so that the oscillating motion of the sound wave outside the immobilized layer tends to carry gas enriched in one component in one direction and gas enriched in the other component in the opposite direction. This is like a bucket brigade: the sound wave corresponds to the people in the brigade, each of whom lifts a bucket full of the first component out of the immobilized layer, moves it in one direction, pours it back in the immobilized layer, fills his/her bucket with the second component there, moves back in the other direction, pours the second component into the immobilized layer, and refills with the first component to repeat the cycle. Experiments and theory described in Swift et al. supra, and Spoor et al., xe2x80x9cThermoacoustic Separation of a Hexe2x80x94Ar Mixture,xe2x80x9d 85 Phys. Rev. Left. 8, pp. 1646-1649 (2000) demonstrate that this mechanism is occurring. The present invention is directed to a practical method and apparatus to employ this mechanism for challenging separations such as isotopes or isomers. Thermoacoustic mixture separation has several advantages over various previous separation methods used for isotopes, isomers, or other difficult cases. It does not require gravity or differences in densities of the two components of the mixture, it can operate entirely at ambient temperature (or at any other single temperature), the inventory of mixture is small, and the hardware is simple and reliable.
Various features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention is directed to a thermoacoustic device for separating a mixture of gases. An elongated duct is provided with first and second ends and has a length that is greater than the wavelength of sound in the mixture of gases at a selected frequency, and a diameter that is greater than a thermal penetration depth in the mixture of gases at the selected frequency. A first acoustic source is located at the first end of the duct to generate acoustic power at the selected frequency. A plurality of side branch acoustic sources are spaced along the length of the duct and configured to introduce acoustic power into the mixture of gases in the duct so that a first gas in the mixture of gases is concentrated at the first end of the duct and a second gas in the mixture of gases is concentrated at the second end of the duct.